It is known that a variety of physiologically active substances are produced by microorganisms and that a Streptomyces strain designated or identified as NCIB 8845 or ATCC No. 14903 produces an antibiotic named actinonin which exhibits an activity inhibitory to aminopeptidase M as well as an immunopotentiating activity [See U.S. Pat. No. 3,240,787 and Umezawa et al., "The Journal of Antibiotics," vol. 38, p. 1629-1630 (1985)]. The activity of actinonin inhibitory to aminopeptidase M is not so strong that its IC.sub.50 value against aminopeptidase M is 0.40 .mu./ml. Thus, there remains a need for an inhibitory agent which is more active than actinonin against aminopeptidase M and which is useful in immunological therapeutic treatment of various diseases in mammalian animals, including men.
An object of this invention is to provide new compounds which are useful as the immunopotentiator. A further object of this invention is to provide processes for the fermentative production of these new compounds.
We, the present inventors, have made extensively our research in an attempt to produce and provide new physiologically active compounds which have a higher activity inhibitory to aminopeptidase M. As a result, we have now found that when a new strain of the genus Streptomyces which was allotted a laboratory designation MH663-2F6 is cultivated in a culture medium, there are produced and accumulated in the culture such new two substances which show the activities inhibitory to aminopeptidase M. We have succeeded in isolation of these two substances and in purification of them. From the chemical, physical and biological studies of these isolated two substances, it has been confirmed that each of these substances is a new compound which is less toxic and which is distinguishable from any of the known compounds. Thus, we have nominated these two new compounds as probestin and prostatin, respectively. Probestin and prostatin have the following chemical structures; ##STR1##